1. Field of the Invention
The present invention relates to a sheet forming (including drawing) method and apparatus wherein a bead capable of applying a variable bead force to a blank (hereinafter, a variable bead) is operated to be effective selectively in position and time and to press a blank in a point manner so that formability of the blank is improved (and, breakage of the blank during sheet forming is essentially prevented).
2. Description of Related Art
In conventional methods of drawing, in order to prevent breakage of a blank during drawing, methods such as selecting a blank material having a high extensibility, increasing a thickness of a blank, and coating lubricating oil of a high lubrication on a blank have been used.
Further, Japanese Patent Publication SHO 59-206120, the disclose of which is hereby incorporated by reference, discloses a longitudinal bead capable of applying a variable bead force to a blank. The force of the longitudinal bead is made less effective at a predetermined stage of the forming time period to improve the formability of the blank.
However, the conventional drawing methods described above have the following problems:
Grading-up the blank material and increasing the blank thickness is accompanied by an increase in manufacturing cost. High lubrication of the oil is accompanied by sliding of the blank relative to the dies, which will generate wrinkling in the blank during drawing and will lower the dimensional accuracy of the blank.
With the longitudinal variable bead, because the bead should be movable relative to either one of a blank holder and an opposing die to which the bead is mounted, it is difficult to embody a movable longitudinal bead structure in a bead mounting die.